麻省理工大学课件：系统微生物学2-单元结构_功能(笔记)

20.106J – Systems MicrobiologyLecture 2Prof. DeLong¾ Reading for next week: Chapter 5 (Biol. Energy, Couscku)¾ Problem Set #1 due next week¾ Reading for today: Purcell, Berg, and Pace• Related to the last lecture:o Life’s history on Earth – EvidencePhylogenetic treeThe same machinery for making proteins – with ribosomes – isused all over Earth.You can map how different the ribosomal RNA is in each species on Earth.• In this way we can compare microbes to eukaryotes.A lot of the Eukarya tree (our own tree) is dominated by microbes – Archaea.Chloroplast RNA falls right next to cyanobacteria on that tree• This supports the endosymbiont hypothesis – chloroplastsderive from cyanobacteria.Similarly, mitochondrial RNA falls by agrobacteria – αproteobacteria.o Life on Earth today: the foundationCO 2/O 2 cycle• To be covered today: Structure, Function, Motilityo The nature of being smallo Cell membranes and cell wallso Flagella• Shape and Appearance – not where the interesting stuff is regarding microbeso They don’t bring in solid food – they bring in dissolved substrates.Surface area to volume:r r r V SA 334432==ππ o “Prokaryote” vs. EukaryoteIn eukaryotes, there are organelles and a nucleus – quite a lot ofcommunication and transport is going on.In prokaryotes, transcription and translation all occur together inthe cytoplasmHowever, “Prokaryote” is in quotes because it is only a negativedefinition – they are defined only by the lack of a membrane-bound nucleus.One group of microbes – Archaea – are a lot more like eukaryotes than they are like bacteria.•Their informational machinery – RNA polymerase,promoters – are more similar to those of eukaryotes.Hence there are Three large branches of life: Bacteria, Archaea,and Eukarya (the two-branch representation of life as prokaryotevs. eukaryote is less accurate).•Cell membranes: phospholipid bilayero Main permeability barriero Embedded integral membrane proteins – communication, transporto Membrane structureBacteria, eukaryotesArchaeao Archaea can still make lipid bilayers – though sometimes they hook them directly together, making a lipid monolayer.This is much more structurally rigid.This is never found in bacteria or eukaryotes.o Membranes act as a protein anchor.o Also energy conservation – protein motive force.o Membrane permeability to various molecules:Simple transport: let a proton down the gradient in order to move things.Group translocation: chemical modification of transportedsubstance driven by phosphoenolpyruvate.ABC system: periplasmic binding proteins are involved and energy comes from ATP.o Transport method:Uniporter: one thing comes in.Antiporter: one thing in, one out.Symporter: two in at once.o Gram-positive bacteria have one phospholipid bilayer.With a thick peptidoglycan layer outside.o Gram-negative bacteria have two bilayersThere is periplasm in between.•Most of the binding proteins are located here.The outer membrane (lipopolysaccharide and protein)•Antibiotic resistance occurs here – resistance thus occursmore easily in gram-negative bacteria.There is a peptidoglycan layer in the middle of the periplasm, but it’s very thin.•It forms a net-like structure, with a single molecule ofpeptidoglycan that acts as a nylon stocking.• This maintains structure, shape, and integrity.Lipopolysaccharide chains outside – can often make people sick o In penicillin, lysozyme chews up peptidoglycanThen water all rushes in, causing lysisPenicillin inhibits the crosslinksTherefore penicillin only works on cells that are growingo Archea – S-layers, pseudo peptidoglycan• Motilityo Flagella – moves like a propeller in bacteria, not like a whip – they’re rigidVideo clip: E. coli moving with rotating flagellaηνρa ForcesViscous Forces Inertial ≈← Fluid Density ← Fluid Viscosity The movement is dominated by viscosityo Clamshell hypothesis: reciprocal motion doesn’t work at low Reynolds number – instead it’s a rotary motorProton motive force turns a ring that drives the motoro Flagella are hollow on the insideMade of one protein: flagellinIt grows from the inside-outVery complexo Going counter-clockwise they drive the cell forwardGoing clockwise, they fly out in a tumbleo By changing the frequency, you get longer or shorter runs。